The casing of a distributor is usually provided with a ventilation opening for the purpose of ventilating the interior thereof, ventilation is needed because the casing is susceptible to deterioration, especially the portion made of resins (deterioration of insulation resistance of the surface of insulating material of resins) and to rust or oxidation at the portion made of metals. These problems are serious and have to be prevented, because they can lead to generation of nitric oxides, nitric acides, etc. inside the casing, due to spark discharges between the rotor arm and the side electric terminals. However this ventilation opening often permits, when it is left open, ingress of water spray during driving of a vehicle on rainy days, with a result of misfiring of the distributor or rust producing in the casing. Conventionally fitting or crowing of a water protecting cap of rubber, having a ventilation passage therein, on the ventilation opening has been practiced. This still does not protect fully against water ingress, because the ventilation passage formed in the cap itself leaves room permitting small amounts of water to come into the casing of the distributor.